Flow in turbomachines such as gas turbine engines is usually unsteady. The unsteady flows can give rise to component (e.g. rotor blade) vibration owing to the resulting unsteady loading on the components and additional unsteadiness in the aerodynamic coupling between components such as neighbouring blades. Initially the unsteadiness takes the form of a small perturbation about the steady flow and is often associated with a small movement of the component from its steady position. When the disturbances in the system are small, we can assume that they are also linear, and as will be shown later, such an assumption can be valid in relation to active control systems and allows simplification of the control systems.
Some examples of unsteady motions in this category will now be described.
(i) Blade Flutter. A review of the general features of turbo-machinery flutter is given by F. O. Carta of USAF Aero Propulsion Laboratory in chapter 22 of the book "The Aerothermodynamics of Aircraft Gas Turbine Engines", edited by G. C. Oates, 1978 (Ser. No. AD/A059784). In flutter, the blades vibrate, and as a result extra aerodynamic forces act on the blades. At certain flow conditions these forces may have a phase relationship with the blades that causes energy to be transferred from the airflow to the blade. Flutter occurs when this energy transfer exceeds the energy dissipated in the structure of the blades, resulting in increasing amplitude of vibration. While flutter may occur for a single blade of the machine, in most cases it involves all the blades. These blades are coupled together either structurally or by the unsteady aerodynamic forces. When the individual blades are identical, vibration of the bladed disc is made up of a number of modes (so-called "assembly modes") in which the blades have fixed phase angle to one another; usually only a small number of these modes is unstable and is involved in flutter.
When the individual blades are non-identical, a set of phenomena collectively known as mistuning become important. The modes described above are disrupted and may be localised at one position on the bladed disc. In general, mistuning increases stability and reduces the susceptibility to flutter. For further information on mistuning, see the paper by Crawley and Hall entitled "Optimisation and Mechanisms of Mistuning in Cascades". ASME 29th Int. Gas Turbine Conf. Paper 84-ST-196, Amsterdam, June 4-7, 1985.
It should be noted that the present invention is applicable to flutter modes of individual blades, for example first flap, second flap, first torsion, second torsion, etc., and also to the assembly modes, involving coupling between blades in the same rotor which give rise to vibrations having particular numbers of nodal diameters. Note that the assembly modes can rotate at speeds other than the disc rotational speed.
(ii) Forced Vibration. Besides the forces due to its self-motion described above, the blades of the rotor are acted on by unsteady aerodynamic forces due to their interaction with upstream or downstream distrubances. These may be due to, for example, distorted inlet flows to an engine, turbulence, or the wakes from upstream blade rows. If the frequency of these aerodynamic forces coincides with the resonant frequency of the blades, large vibration amplitudes may result. The vibrational modes of the bladed disc in this phenomenon are similar to those for flutter. Mistuning can also be important. In this case it often increases the vibration levels, see for example MacBain and Whaley, "Maximum Resonant Response of Mistuned Bladed Discs" in "Vibration of Bladed Disc Assemblies", p. 153-160, ASME 1983, Edited by D. J. Ewins. and A. V. Srinivason.
(iii) Surge and Rotating Stall. These phenomena are discussed in an article by Greitzer, "Axial Compressor Stall Phenomena", ASME J. Fluids Eng. 102,134-151, June 1980. When a compressor is run at constant speed and has its load increased, a point is reached at which the flow becomes unstable and the pressure rise through the compressor reaches a maximum. The instability can either be in the form of surge or rotating stall. In the former, the flow oscillates in a predominantly axisymmetric manner dependent on the characteristics of the compressor and its inlet and exit flow systems. In rotating stall, there are a number of `stall-cells`, which are patches where the flow over several rotor blades is stalled. These rotate at roughly half the speed of the compressor rotor. A paper by Moore, "A Theory of Rotating Stall of Multistage Axial Compressors", Parts I-III, ASME 28th Int. Gas Turbine Conf., Paper 83-GT-44,45,46, Phoenix, Arizona, Mar. 27-31, 1983, shows that at their inception, both these phenomena are linear.
(iv) Acoustic Resonance. The reader is referred to a paper by Parker and Stoneman, "Acoustically Excited Vibration of Compressor Blades", Int. Conf. on Vibrations in Rotating Machinery, Paper C 321-84, York, England Sept., 1984. This phenomenon arises from a resonance of the air inside a blade row and is associated with an unsteady motion of the air flow that travels round the annulus of the machine at nearly sonic speed. The resonance can be self-excited by vortex shedding from the trailing edges of the blade row at resonance. The vortex shedding couples to the resonance to produce large amplitudes of vibration which can cause structural failure. The resonance is strongly affected by the geometry of the upstream and downstream flows and by the spacing between blade rows. Again it must be linear at its inception.
Clearly, acoustic resonance as defined here requires the blades to be contained within a surrounding casing and is most likely to arise in turbocompressors and turbines of the axial flow types.
The unsteady motion phenomena mentioned above cause particular problems in the operation and control of turbomachines, particularly gas turbine aeroengines. Stated in the most general way, the main problematic effect of these phenomena is to cause the operating range of the turbomachine in which they occur to be restricted--e.g. by means of special scheduling of the main engine fuel control system--so as to avoid troublesome conditions caused by the phenomena.
Much previous research has concentrated on devising control systems which enable turbomachinery to operate over a wider range of conditions than would otherwise be possible, the control systems involving sensing a threshold level of a condition in the turbomachine indicative of the onset of a troublesome unsteady motion phenomenon and taking corrective action in the form of initiating a step change in an operating characteristic of the turbomachine whenever the threshold level is exceeded. This approach is illustrated by U.S. Pat. No. 4,196,472 to Ludwig et al, which relates to a system for controlling rotating stall in axial flow compressors of turbopropulsors. For a full appreciation, the reader is referred to the patent, but the control system therein described may be characterised as operation by sensing unsteady pressures in the compressor, filtering and rectifying them, generating a control signal from the resultant pressure signals whenever they exceed a threshold value indicative of the onset of rotating stall, and using the control signal to actuate means producing a step change in a compressor characteristic which eliminates the imminent rotating stall. The step change in a compressor characteristic may be produced by such means as opening a compressor bleed door, adjusting the angle of stator vanes, or even altering exhaust nozzle area or fuel flow to the combustor.
Whilst the Ludwig et al control system would seem to be effective in enabling reduction or elimination of stall safety margins, thus enabling beneficial gains in engine performance, control systems enabling even further gains to be made would be desirable.
One line of research which has been noted by the present applicants concerns continuous active suppression of flutter and alleviation of gust effects on aircraft wings. See for example Meirovitch and Silverberg, "Active Vibration Suppression of a Cantilever Wing", J. Sound and Vibration 97, 489-498, 1984. There, a feedback system was used to drive control surfaces on the wing in such a manner as to reduce the vibration amplitude, the control surfaces being in motion at a frequency approximately the same as the vibration in the wing, but the motions being such as to counter the effects of the vibrations, thereby enabling the wing to perform better under extreme conditions.
As mentioned previously, the available operating ranges of turbomachines, as exemplified by gas turbine aeroengines, is limited by blade flutter, by other instabilities such as rotating stall and surge, by vibration due to distorted flows, and by acoustic resonance. It occurred to the present applicants, even though the dynamics of turbomachines differ conceptually from those of an aircraft wing, that suitable fast active control systems could perhaps be used to give continuous active control of these unsteady motion phenomena, thereby increasing the operating range of engines and enabling relaxation of constraints on aerodynamic and mechanical design. In turn, this would enable designs of greater aerodynamic efficiency and less weight.
Consequently, it is an object of the present invention to enable an increase in the range of operating conditions of turbomachinery.
It is an allied object of the invention to maximise performance gains in respect of turbomachines.